The present invention relates generally to stabilized sulforaphane, and analogs thereof. The present invention further relates generally to methods of stabilizing sulforaphane and analogs thereof.
Isothiocyanates, such as phenethyl isothiocyanate (PEITC) and sulforaphane, have been shown to inhibit carcinogenesis and tumorigensis and as such are useful chemopreventive agents against the development and proliferation of cancers. These compounds work on a variety of levels. Most notably, they have been shown to inhibit carcinogenesis through inhibition of cytochrome P450 enzymes, which oxidize compounds such as benzo[a]pyrene and other polycyclic aromatic hydrocarbons (PAHs) into more polar epoxy-diols which can then cause mutation and induce cancer development. Phenethyl isothiocyanate (PEITC) has been shown to induce apoptosis in certain cancer cell lines, and in some cases, is even able to induce apoptosis in cells that are resistant to some currently used chemotherapeutic drugs.
Sulforaphane, as discussed above, is known as an anticancer and antimicrobial compound found in cruciferous vegetables such as cabbage, broccoli, broccoli sprouts, brussel sprouts, cauliflower, cauliflower sprouts, bok choy, kale, collards, arugula, kohlrabi, mustard, turnip, red radish and watercress. In the plant, it is present in bound form as glucoraphanin, a glucosinolate. Sulforaphane is often formed from glucoraphanin on plant cell damage via an enzymatic reaction.
Various synthetic methods of producing sulforaphane are known in the art. Sulforaphane was synthesized as early as 1948 by Schimd and Karrer (Schimd H. and Karrer, P.; Helvetica Chimica Acta. 1948; 31; 6: 1497-1505). The Schimd synthesis results in a racemic mixture. Other methods of synthesizing sulforaphane developed since 1948 also tend to result in racemic mixtures of sulforaphane. Additionally, sulforaphane is known as an unstable oil. Due to its instability, sulforaphane is difficult to manufacture and distribute.
Cyclodextrins are a family of cyclic oligosaccharides composed of 5 or more α-D-glucopyranoside units linked 1-4. The largest well-characterized cyclodextrin contains 32 1,4-anhydroglucopyranoside units, while (as a poorly characterized mixture) 150 membered cyclic oligosaccharides (and greater) are also known.
Cyclodextrins are able to form host-guest complexes with hydrophobic molecules given the unique nature imparted by their structure. Cyclodextrins include an exterior that is sufficiently hydrophilic to impart water solubility to the cyclodextrin. The interior of the cyclodextrin is known to be hydrophilic, but can be considered hydrophobic with respect to the exterior of the cyclodextrin.
The natural cyclodextrins, in particular beta-cyclodextrin, have limited aqueous solubility and their complex formation with lipophilic drugs often results in precipitation of solid drug-cyclodextrin complexes. Thus, the solubility of beta-cyclodextrin in water is only about 18.5 mg/mL at room temperature. This low aqueous solubility is, at least partly, associated with strong intramolecular hydrogen bonding in the cyclodextrin crystal lattice. Substitution of any of the hydrogen bond-forming hydroxyl groups, even by hydrophobic moieties such as methoxy groups, will increase the aqueous solubility of beta-cyclodextrin. In addition, since these manipulations frequently produce large numbers of isomeric products, chemical modification can transform the crystalline cyclodextrins into amorphous mixtures increasing their aqueous solubility.
Cyclodextrin derivatives of current pharmaceutical interest include the hydroxypropyl derivatives of alpha-, beta- and gamma-cyclodextrin, sulfoalkylether cyclodextrins such as sulfobutylether beta-cyclodextrin, alkylated cyclodextrins such as the randomly methylated beta-cyclodextrin, and various branched cyclodextrins such as glucosyl- and maltosyl-beta-cyclodextrin (T. Loftsson and M. E. Brewster, “Cyclodextrins as pharmaceutical excipients”, Pharm. Technol. Eur., 9(5), 26-34 (1997); T. Loftsson and M. E. Brewster, “Pharmaceutical applications of cyclodextrins. I. Drug solubilization and stabilization”, J. Pharm. Sci. 85(10), 1017-1025 (1996); R. A. Rajewski and V. J. Stella, “Pharmaceutical applications of cyclodextrins. 2. In vivo drug delivery”, J. Pharm. Sci. 85(11), 1142-1169 (1996); T. Irie and K. Uekama, “Pharmaceutical applications of cyclodextrins. 3. Toxicological issues and safety evaluation”, J. Pharm. Sci., 86(2), 147-162 (1997); V. J. Stella and R. A. Rajewski, “Cyclodextrins: their future in drug formulation and delivery”, Pharm. Res., 14(5), 556-567 (1997); T. Loftsson, “Increasing the cyclodextrin complexation of drugs and drug bioavailability through addition of water-soluble polymers”, Pharmazie, 53, 733-740 (1998)).
In aqueous solutions, cyclodextrins form complexes with many drugs through a process in which the water molecules located in the central cavity are replaced by either the whole drug molecule, or more frequently, by some lipophilic portion of the drug structure. Once included in the cyclodextrin cavity, the drug molecules may be dissociated through complex dilution, by replacement of the included drug by some other suitable molecule (such as dietary lipids or bile salts in the GI tract) or, if the complex is located in close approximation to a lipophilic biological membrane (such as the mucosal membrane of the GI tract), the drug may be transferred to the matrix for which it has the highest affinity. Importantly, since no covalent bonds are formed or broken during the drug-cyclodextrin complex formation, the complexes are in dynamic equilibrium with free drug and cyclodextrin molecules (R. A. Rajewski and V. J. Stella, “Pharmaceutical applications of cyclodextrins. 2. In vivo drug delivery”, J. Pharm. Sci. 85(11), 1142-1169 (1996)).
Various methods have been applied to the preparation of drug-cyclodextrin complexes (T. Loftsson and M. E. Brewster, “Pharmaceutical applications of cyclodextrins. I. Drug solubilization and stabilization”, J. Pharm. Sci. 85(10), 1017-1025 (1996); T. Loftsson and M. E. Brewster, “Cyclodextrins as pharmaceutical excipients”, Pharm. Technol. Eur., 9(5), 26-34 (1997)). In solution, the complexes are usually prepared by addition of an excess amount of the drug to an aqueous cyclodextrin solution. The suspension formed is equilibrated (for periods of up to one week at the desired temperature) and then filtered or centrifuged to form a clear drug-cyclodextrin complex solution. Since the rate determining step in complex formation is often the phase to phase transition of the drug molecule, it is sometimes possible to shorten this process by formation of supersaturated solutions through sonication followed by precipitation.
For preparation of the solid complexes, the water may be removed from the aqueous drug-cyclodextrin solutions by evaporation or sublimation, e.g. spray-drying or freeze-drying. Other methods can also be applied to prepare solid drug-cyclodextrin complexes including kneading methods, co-precipitation, neutralization and grinding techniques. In the kneading method, the drug is added to an aqueous slurry of a poorly water-soluble cyclodextrin such as beta-cyclodextrin. The mixture may be thoroughly mixed, often at elevated temperatures, to yield a paste which is then dried. This technique can frequently be modified so that it can be accomplished in a single step with the aid of commercially available mixers which can be operated at temperatures over 100° C. and under vacuum. The kneading method is a cost-effective means for preparing solid cyclodextrin complexes of poorly water-soluble drugs. Co-precipitation of a cyclodextrin complex through addition of organic solvent is also possible. Unfortunately, the organic solvents used as precipitants often interfere with complexation which makes this approach less attractive than the kneading method. It has been discovered that some organic solvents under some specific conditions, e.g. 10% (v/v) aqueous acetic acid solution, can enhance the complexation. Solid complexes of ionizable drugs can sometimes be prepared by the neutralization method wherein the drug is dissolved in an acidic (for basic drugs) or basic (for acidic drugs) aqueous cyclodextrin solution. The solubility of the drug is then lowered through appropriate pH adjustments (i.e. formation of the un-ionized drug) to force the complex out of solution. Finally, solid drug-cyclodextrin complexes can be formed by the grinding of a physical mixture of the drug and cyclodextrin and then heating the mixture in a sealed container to 60 to 90° C.